Since the 1980s, several hundred tiles– all roughly the size of an American state license plate, and all bearing roughly the message above– have been found embedded in the pavement of roads in streets in two dozen major U.S. cities and four South American capitals.

There’s no consensus among scholars of the tiles as to their reference or meaning. It’s pretty widely held that the “Toynbee” reference is to historian Arnold Toynbee, perhaps to a passage (in Experiences):

Human nature presents human minds with a puzzle which they have not yet solved and may never succeed in solving, for all that we can tell. The dichotomy of a human being into ‘soul’ and ‘body’ is not a datum of experience. No one has ever been, or ever met, a living human soul without a body… Someone who accepts – as I myself do, taking it on trust – the present-day scientific account of the Universe may find it impossible to believe that a living creature, once dead, can come to life again; but, if he did entertain this belief, he would be thinking more ‘scientifically’ if he thought in the Christian terms of a psychosomatic resurrection than if he thought in the shamanistic terms of a disembodied spirit.

Others suggest that the tiles allude to Ray Bradbury’s story “The Toynbee Convector,” to Arthur C. Clarke’s story “Jupiter V,” or– perhaps, given the direct 2001 reference, most likely– to Stanley Kubrick’s film (in which, readers will recall, hibernating astronauts who had secret training were to be revived upon arrival on Jupiter).

And while there’s no agreement on the identity of the tiler, a majority of enthusiasts believe that “he” is from Philadelphia– both because the City of Brotherly Love hosts the highest concentration of the plaques and because a collection of tiles found there deviate from the norm to ascribe a plot to John S. Knight (of Knight-Ridder, the erst-while newspaper publishers), the Mafia, and others.

See a (nearly) complete list of tiles and their locations here, a set of photos here, and learn how they are implanted here. Visit this site for a peek at a Sundance award-winning documentary on the Tiles.

As we watch where we’re walking, we might recall that it was on this date in 1504 that Michelangelo’s 17-foot-tall marble David was unveiled in a public square outside the Palazzo della Signoria, the seat of civic government in Florence.